Like other metrology technologies, X-ray computed tomography (CT) systems must be traceably calibrated in order to deliver true metrological performance. Many of the errors affecting CT-systems need an adequate calibration of the machine reducing systematic errors. Although being in the pending process of developing calibration standards, there is currently no internationally recognized standardized procedure for calibrating X-ray CT metrology equipment. Instead, calibration must be derived and traced from another metrology method having such a standard, for example, an optical scanner or a touch probe attached to a coordinate measuring machine arm (CMM arm).
A preferred calibration method should be relatively low-cost and simple, and therefore relies on having a traceable calibration piece consisting of a number of spheres, which are chosen to have scale and X-ray properties similar to those of the test piece.
Typically the centre-to-centre distances of the spheres of calibration piece are measured by the X-ray CT instrument and compared with measurement data of the calibration piece obtained by another metrological method. Using spheres is absolutely required, since a linear threshold dimension would compromise accuracy.
Sphere gauges are well known from the art as calibration artefact, and are particularly recommended for presenting a minimum deviation of the measured length. Their spherical shape provides a self-centring line of contact. Results of experiments on the use of dedicated reference calibration artefacts are published, and according to the paper “Analysis of the error sources of a Computer Tomography Machine”, by V. Andreu et al., a 27-sphere gauge shows to be very suitable to determine e.g. size measurement error because it is measured at different heights and the error appears to be independent on the position of the interest region on the detector.
Having measured the calibration piece, the exact scaling of the CT data can be performed so that it matches actual part dimensions and the voxel size of the 3D model being set to the value found from the traceable calibration. A traceable calibrated 3D CT-model is delivered after mapping with a true local surface determination.
Some designs in the prior art employ multiple styli with ruby spheres mounted onto a heavyweight block; this calibration artefact is rather large to store or to transport for executing calibration tests in the field. Because of the mounted styli, being thin, the calibration piece is also of a fragile construction, particularly difficult when packaging or transportation is required. Furthermore, assembly of this piece is prone to error, the large range of different heights adding an extra dimension to the calibration requirement, providing probe access problems.
DE 10 2005 033187 describes a two plate calibration artefact that stands vertically on a turntable. With the arrangement of balls, beam hardening is inevitable, and its thickness implies a limited useful in terms of calibrating several parameters. DE 10 2006 028 452 describes a calibration block machined from solid metal and provided with a plurality of surface calibration indentations. The necessary thickness also means a limited usefulness in terms of calibrating several parameters. DE 10 2006 014 509 describes a calibration block for a co-ordinate measurement machine.
The gauges for calibration and/or verification of the art have one fixed set-up that obliges use of the format with specifications as given, and does not allow for any adjustment or fine-tuning during the calibration procedure. Moreover, the geometry of X-ray CT instruments vary depending on the application, necessitating a plurality of different artefacts.
Therefore, there is a need for a more flexible artefact.